The use of a generator of high energy neutrons has been employed for a long time for neutron-gamma ray or neutron-neutron well logging. A neutron generator has advantages compared with chemical neutron sources, in particular it features a negligible amount of radiation other than the desired neutrons; a controllable yield of neutrons in bursts or continuously; neutrons at higher energies than are easily accessible with a chemical source; mono-energetic neutrons; and control of the generator so as to permit its deactivation prior to withdrawal from or insertion in a well. The ability to deactivate the neutron generator by turning it off is an advantageous safety feature, compared to a chemical source which is always on. The first five of these attributes are important in obtaining more informative logs, while the last is valuable in minimizing health hazards to operating personnel.
Neutron generators used in oil well logging tools usually require controlled low pressure atmospheres. Neutron generators usually have three major features: (i) a gas source to supply the reacting substances, such as deuterium (H2) and tritium (H3); (ii) an ion source comprising usually at least one cathode and an anode; electrons are emitted from the cathode surface when an electrical impulse is applied to the anode; impact of the primary electrons on the gas molecules result in subsequent secondary electrons being stripped from the gas molecules, thus generating positively charged ions; and (iii) an accelerating gap which impels the ions to a target with such energy that the bombarding ions collide with deuterium or tritium target nuclei in neutron-generating nuclear fusion reactions.
Ordinarily, negative electrons and positively charged ions are produced through electron and neutral gas molecule ionizing collisions within the ion source. Electrodes of different electrostatic potential contribute to ion production by accelerating electrons to energy higher than the ionization threshold. Collisions of those energetic electrons with gas molecules ionize the latter, thereby producing ions and additional electrons.
Referring to FIG. 1, a hot cathode neutron generator (100) is shown, such as that described in U.S. Pat. No. 5,293,410, the contents of which are herein incorporated by reference in its entirety. The hot cathode generator (100) achieves neutron production via a fusion reaction produced when deuterium and/or tritium ions are electrostatically accelerated into a target (125) also comprising deuterium (D) and/or tritium (T). The hot cathode neutron generator (100) comprises a hermetically-sealed tube (128) enclosing a filament (105), a cathode (110), a gas (112), an extractor (120), and a target (125). In one or more embodiments, the filament (105) comprises zirconium, which serves as a deuterium or tritium reservoir. Passing an electric current through the filament provides a gas of deuterium or tritium within the hermetically-sealed tube (128). In one or more embodiments, the gas is diatomic, for example, D2. Passing a current through the cathode (110) liberates electrons (112) which are accelerated by the grid (115) and gain kinetic energy. Pulsed operation may be achieved by applying a burst of positive voltage to the grid (115) to energize the electrons (112) from the cathode (110) to ionization energies. The energized electrons (112) ionize the neutral gas locally. The ions are then accelerated toward the target (125) by the potential difference between the extractor (120) and the target (125). The ions accelerating toward the target may be referred to as a beam (122) or ion beam. In the hot cathode neutron generator (100), the monatomic fraction of the ion beam (122) created by electronic collisions with neutral atoms may be small, perhaps 5% or less, with the remainder being diatomic. The monatomic fraction of an ion beam is the ratio of ions composed of a single atom to the total number of ions, both single-atom and multi-atom (diatomic and/or triatomic, typically). If, for example, the neutral gas were H2, the monatomic fraction of the ion beam would equal the ratio of the number of H+ ions to the sum of H+ and H2+ and H3+ ions. The same would apply to a neutral gas of D2 or T2. Because the mass of the diatomic ion is double that of the monatomic ion, it is accelerated to only half the energy per deuteron. Further, with an accelerating potential of about 100 kV, the monatomic ion beam yields proportionally about three times more neutrons than the diatomic ion beam, since the nuclear fusion cross section is energy dependent.